Clean up pg_query gem native extension directory

This removes a 30 MB directory that contains the lib_pgquery source and compiled objects. These files are not needed for the pg_query gem to work.

After deleting the files:

$ sudo gitlab-rails c
--------------------------------------------------------------------------------
 GitLab:       13.5.0-pre (9bc30dc1dd1) EE
 GitLab Shell: 13.8.0
 PostgreSQL:   11.9
 Geo enabled:  yes
 Geo server:   primary
--------------------------------------------------------------------------------
require 'pg_query'
Loading production environment (Rails 6.0.3.3)
irb(main):001:0> require 'pg_query'
=> true
irb(main):002:0> PgQuery.normalize("SELECT * FROM users WHERE user_id = 1")
=> "SELECT * FROM users WHERE user_id = $1"

Closes #5747 (closed)