Remove completion scripts I hardly ever use
Add hack to remove git=hub alias before running __git_ps1

The alias makes git PS1 a lot slower with the configured GIT_* niceties
for the PS1, taking around 1s+ to return each time when in a git repo.

Unaliasing before running __git_ps1 results in around 0.6~0.7 seconds
faster response from the command, improving the usability considerably.

$ time __git_ps1
 (master>)
 real 0m1.031s
 user 0m0.698s
 sys  0m0.312s

$ unalias git

$ time __git_ps1
  (master>)
  real  0m0.408s
  user  0m0.274s
  sys 0m0.125s

A final solution would be to change the default __git_ps1 function to
use "command git" instead, which would bypass any alias for that command.

More info: https://github.com/github/hub/issues/254