tests: drop pip test on the client

The PIP testing is fragile, various hacks are included already. It has
been failing persistently in the CI and locally on OpenSuSE. Since
general PIP use requires network access, it is better done during CI
anyway.

Differential Revision: https://phab.mercurial-scm.org/D9928
13 jobs for topic/default/pip-test in 22 minutes and 6 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Tests
passed #159632
checks-py2

00:02:38

passed #159633
checks-py3

00:03:24

passed #159634
rust-cargo-test-py2

00:02:19

passed #159635
rust-cargo-test-py3

00:02:20

passed #159636
test-py2

00:19:15

passed #159642
test-py2-chg

00:19:25

passed #159638
test-py2-pure

00:19:00

passed #159640
test-py2-rust

00:21:36

passed #159637
test-py3

00:11:39

passed #159643
test-py3-chg

00:08:03

passed #159639
test-py3-pure

00:11:01

passed #159641
test-py3-rust

00:13:33

 
  Phabricator
passed #159644
phabricator-refresh

00:00:28