Remove Build-Ubuntu18-GCC job

This partially reverts
https://skia-review.googlesource.com/c/skia/+/254797. The Ubuntu18 GCC
job was added in anticipation of using it for Valgrind jobs; however,
https://skia-review.googlesource.com/c/skia/+/255537 switches Valgrind
to Clang, which makes this unnecessary.

Bug: skia:9632
Change-Id: I97d7e0d4e3be109ae48b386140117aac4279618f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/255790
Commit-Queue: Ben Wagner aka dogben <benjaminwagner@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
8 files changed