git-gui: remove procs gitexec and _git_cmd

git-gui: remove procs gitexec and _git_cmd

gitexec looks up and caches the method to execute git subcommands using
the long deprecated dashed form if found in $(git--exec-path). But,
git_read and git_write now use the dashless form, by-passing gitexec.
This leaves two remaining uses of gitexec: one during startup to define
use of an ssh_key helper, and one in the about dialog box. These are
neither performance critical nor likely to be called more than once, so
do not justify an otherwise unused cacheing system.

Let's change those two uses, making gitexec unused. This allows removing
gitexec and _git_cmd.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>