git-gui: Remove explicit references to 'aspell' in message strings

git-gui: Remove explicit references to 'aspell' in message strings

Users may or may not be using aspell here.  About the only thing
we are using that is aspell specific (and not supported by ispell
or an ispell variant) is some command line options when we start
up aspell, and a forced encoding of UTF-8.  Both of these can be
corrected and/or cleaned up by users through an aspell wrapper
script, or through further improvements to git-gui.  There is no
reason to require our translated strings to reference a specific
spell checker, especially if that spell checker implementation is
not very suitable for the language being translated.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>