Delete stray file from git-extras on buildbots

[Rainer Müller]

Hi Bill,

there is a stray file on the build bots for Lion and Snow Leopard as
git-extras wrote to the prefix directly instead of to the destroot.
Can you please delete '/opt/local/etc/bash_completion.d/git-extras'?

Failing builds:
https://build.macports.org/builders/buildports-lion-x86_64/builds/15320
https://build.macports.org/builders/buildports-snowleopard-x86_64/builds/21668

Rainer