remove .buckversion files from fbsource/xplat

Summary: Buck has not relied on the .buckversion file for a while now. I am trying to clean up the number of configs at the root of the cell for buck. This diff attempts to remove .buckversion code referecnes from fbsource/xplat Instead of calling cat .buckversion to get the buckversion hash, you can call buck --fast-version which parses the buck-java11 file without downloading buck. Alternatively, you can also do something like cat .buck-java11 | grep -o -E -e "[0-9a-f]{40}" | head -1  to get the buckversion hash.

Reviewed By: stepancheg

Differential Revision: D28579639

fbshipit-source-id: 6231e16df41f3e403098576e4bfd5d5a2fd38a14

.gitignore

@@ -5,10 +5,7 @@
 /.buckconfig.local
 /.buckd
 /gentest/test.html
-.buckversion
-.cxx
-.idea
-/local.properties
+.buck-java11
 
 # Jekyll
 /.sass-cache/