Remove standalone build mode
Before this change, gc-conductor defined its own version of jgit used by
gc-executor. With this change, gc-conductor uses the same jgit version
used by gerrit core, as the only build mode supported is in Gerrit tree.
This removal is possible because Gerrit uses jgit as a submodule since
v3.1.
Therefore, the commands used to build gc-executor are adapted to reflect
how to build gc-executor in Gerrit tree.
LFS plugin had a similar change [1] and uses the same jgit version as
core's since stable-3.1.
[1] 2acd29ba7d82375656acf7be5fe172554a6c957a
Change-Id: I2f58bda270b08885c08fc64693a56610f64dcbef
19 files changed
