.gitignore: Remove duplicate .cache ignore entries.

After clangd moved its generated index files from .clangd/index to
.cache/clangd/index[1], 3 separate CLs added .cache entries to the
.gitignore. Remove duplicate entries so that we only keep one entry.

[1]: https://github.com/llvm/llvm-project/commit/9b55bc4d1197aedd76b0f4e04270265cb74cd0b8

Change-Id: I8cb9cda444f5230001df03949c4ee3bc89b7b58e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315763Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791385}

.gitignore

@@ -58,7 +58,6 @@ tags
 Thumbs.db
 v8.log
 vs-chromium-project.txt
-/.cache/
 /.clangd/
 /.clangd-index/
 # Settings directories for eclipse
@@ -71,7 +70,6 @@ vs-chromium-project.txt
 /android_emulator_sdk
 /ash/ash_unittests_run.xml
 /base/base_unittests_run.xml
-/.cache/
 # See build/.gitignore for entries covering src/build.
 
 # The Chrome OS build creates a /c symlink due to http://crbug.com/54866.
@@ -330,4 +328,3 @@ vs-chromium-project.txt
 # Ignore the default results output directory for tools/run-swarmed.py
 /results
 
-.cache