Drop non gzipped sourcemap files

This saves us about 17MB in packagesize. If we had dropped all
non-gzipped JS we would have gained an additional 6MB in savings, but we
currently allow admins to turn off gzip in nginx, and making the
decision whether its safe to change the other gzip files is harder.

changelogs/unreleased/drop-uncompressed-sourcemap.yml

+---
+title: Removed non-gzipped files for sourcemaps to save on package size
+merge_request: 3592
+author:
+type: changed

config/software/gitlab-rails.rb

@@ -171,6 +171,9 @@ build do
   delete 'tmp'
   delete 'public/uploads'
+  # Drop uncompressed sourcemap files. We will keep the gziped versions.
+  command "find public/assets/webpack -name '*.map' -type f -print -delete"
+
   # Cleanup after bundle
   # Delete all .gem archives
   command "find #{install_dir} -name '*.gem' -type f -print -delete"