Drop --quiet argument from some git commands

The method _git_command() captures both stderr and stdout, and never
returns it to the caller. It's only used internally in order to display
the error message, in case there's an error. The logic is to show stderr
if it's not empty, and otherwise falls back to displaying stdout.

So it seems that we'd better NOT use '--quiet' with _git_command(): it
doesn't seem to be needed, and it can even cause git to NOT show any
error message (that's the case when 'git fetch --tags' fails, for
example).

Therefore this commit removes '--quiet' in places where it was passed to
_git_command().

Note that I *think* we could also remove it from the clone() method,
but there's https://bugs.debian.org/886992, so I prefer not to touch it.

Note that the method remove_files() doesn't seem to be used anywhere,
neither in gbp itself, neither outside of gbp, per:
* https://codesearch.debian.net/search?q=import+gbp&literal=1&perpkg=1
* https://codesearch.debian.net/search?q=from+gbp+import&literal=1&perpkg=1

So I took the liberty to drop the 'verbose' keyword argument from this
method.