Remove useless grenad merging

[Kerollmops]

This PR must be merged after #454.

This PR removes the part of code that was merging all of the grenad Readers merging that we don't need as the indexer should have merged them and, therefore, we should only have one final grenad Reader. We reduce the amount of CPU usage and memory pressure we were doing uselessly.

@ManyTheFish are you sure I can skip merging the word_docids database?

Here is the benchmark comparison with the previously merged PR #454:

group                                              indexing_reintroduce-appending-sorted-values_c05e42a8    indexing_remove-useless-grenad-merging_d5b8b5a2
-----                                              -----------------------------------------------------    -----------------------------------------------
indexing/Indexing movies with default settings     1.06      16.6±1.04s        ? ?/sec                      1.00      15.7±0.93s        ? ?/sec
indexing/Indexing songs with default settings      1.16      60.1±7.07s        ? ?/sec                      1.00      51.7±5.98s        ? ?/sec
indexing/Indexing songs without faceted numbers    1.06      55.4±6.14s        ? ?/sec                      1.00      52.2±4.13s        ? ?/sec

And the comparison with multi-batch indexing before #436, we can see that we gain time for benchmarks that index datasets in multiple batches but there is so much variance that it's not clear.

group                                                             indexing_benchmark-multi-batch-indexing-before-speed-up_45f52620    indexing_remove-useless-grenad-merging_d5b8b5a2
-----                                                             ----------------------------------------------------------------    -----------------------------------------------
indexing/Indexing geo_point                                       1.07       6.6±0.08s        ? ?/sec                                 1.00       6.2±0.11s        ? ?/sec
indexing/Indexing songs in three batches with default settings    1.12      57.7±2.14s        ? ?/sec                                 1.00      51.5±3.80s        ? ?/sec
indexing/Indexing songs with default settings                     1.00      47.5±2.52s        ? ?/sec                                 1.09      51.7±5.98s        ? ?/sec
indexing/Indexing songs without any facets                        1.00      43.5±1.43s        ? ?/sec                                 1.12      48.8±3.73s        ? ?/sec
indexing/Indexing songs without faceted numbers                   1.00      47.1±2.23s        ? ?/sec                                 1.11      52.2±4.13s        ? ?/sec
indexing/Indexing wiki                                            1.00    917.3±30.01s        ? ?/sec                                 1.09    998.7±38.92s        ? ?/sec
indexing/Indexing wiki in three batches                           1.09   1091.2±32.73s        ? ?/sec                                 1.00    996.5±15.70s        ? ?/sec

What do you think @irevoire? Should we change the benchmarks to make them do more runs?

[ManyTheFish]

@Kerollmops

@ManyTheFish are you sure I can skip merging the word_docids database?

yes, you can! Below the code that merges this database:
https://github.com/meilisearch/milli/blob/main/milli/src/update/index_documents/extract/mod.rs#L89-L97

[Kerollmops]

bors merge

[bors]

Build succeeded:

• Run Rustfmt
• Tests on macos-latest with stable
• Tests on ubuntu-18.04 with stable
• Tests on windows-latest with stable