Revert "java: move CredentialProvider up to ClientSettings (#1251)"

[pongad]

This reverts commit 2da784d.

[garrettjonesgoogle]

LGTM

[codecov-io]

Codecov Report

Merging #1317 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #1317      +/-   ##
============================================
- Coverage     86.48%   86.47%   -0.01%     
  Complexity     3836     3836              
============================================
  Files           378      378              
  Lines         15328    15323       -5     
  Branches       1694     1694              
============================================
- Hits          13256    13251       -5     
  Misses         1622     1622              
  Partials        450      450

Impacted Files	Coverage Δ	Complexity Δ
.../transformer/java/JavaGapicSurfaceTransformer.java	98.11% <100%> (-0.03%)	43 <0> (ø)

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