build: Remove phpcs from internal-serialization package

The code is covered by phpcs in the root composer.json
Maintain a different (and old) version here is confusing

Change-Id: I9029b55ec4af95013abe2c3a5d8aaf49e4956898

Author: umherirrender

.gitattributes

@@ -4,8 +4,6 @@
 .travis.yml export-ignore
 composer.json export-ignore
 docs export-ignore
-phpcs.xml export-ignore
-.phpcs.xml export-ignore
 phpmd.xml export-ignore
 phpunit.xml.dist export-ignore
 tests export-ignore