DOCS: remove 'git pull'

[mattip]

continuation of gh-1105

Remove git pull, which was meant to solve a possible race condition (if between the time the CI run clones the repo, builds the project, builds the docs, and check out the gh-pages branch someone else has pushed a change to the branch), but does not work.

Please trigger a nightly build after merging this to test it out.

[zhangguanheng66]

Let's merge it after the CI tests are OK

[codecov]

Codecov Report

Merging #1111 (41c9b0f) into master (aa7176b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1111   +/-   ##
=======================================
  Coverage   77.54%   77.54%           
=======================================
  Files          45       45           
  Lines        3086     3086           
=======================================
  Hits         2393     2393           
  Misses        693      693           

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[mthrok]

Triggered nightly build

[mattip]

... and it failed. The gh-pages branch does need to be pulled before pushing.

To https://github.com/pytorch/audio.git
 ! [rejected]        gh-pages -> gh-pages (fetch first)
error: failed to push some refs to 'https://github.com/pytorch/audio.git'
hint: Updates were rejected because the remote contains work that you do
hint: not have locally. This is usually caused by another repository pushing
hint: to the same ref. You may want to first integrate the remote changes
hint: (e.g., 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

I will try again in another PR, sorry.

[mattip]

Note there is no harm in the current state, since the doc push fails it should not break anything.