Drop custom jemalloc library injection

We are now baking jemalloc straight into the Ruby binary as part of gitlab-build-images, so we don't need to use the LD_PRELOAD workaround anymore.

This was also causing issues with nodejs trying to allocate memory during asset compilation.

I verified that jemalloc is still operating and accessible by the app:

git@gitlab-rails-web:~/gitlab$ bundle exec rbtrace -p $(pgrep -f 'worker 0') -e 'Gitlab::Memory::Jemalloc.stats.first(50)'
*** run `sudo sysctl kernel.msgmnb=1048576` to prevent losing events (currently: 16384 bytes)
*** attached to process 89
>> Gitlab::Memory::Jemalloc.stats.first(50)
=> "{\n\t\"jemalloc\": {\n\t\t\"version\": \"5.2.1-0-gea6b3e973b"
*** detached from process 89

Closes #85 (closed)