gitserver: Improve perf by not specifying git protocol version

Created by: mrnugget

(@keegancsmith and I just ran into this when trying to find out why visiting the page of a repository was so slow)

The blog post mentioned in the code that's being removed here got an update in December 2018 that specifies:

Previously the example above used the more rarely used "git ls-remote"
command. A recent change to improve the correctness of that command's
output causes it to request more references.

This change causes git ls-remote with protocol.version=2 to be consistently slower than the variant without a protocol specified.

In order to improve performance, we remove the code and use the older protocol for ls-remote.

Created by: codecov[bot]

Codecov Report

Merging #4350 into master will decrease coverage by 0.01%. The diff coverage is n/a.

Impacted Files	Coverage Δ
cmd/gitserver/server/serverutil.go	49.15% <ø> (-0.29%)
pkg/httpcli/client.go	42.42% <0%> (-6.91%)
cmd/repo-updater/repos/github.go	59.32% <0%> (-0.18%)