gitk: do not invoke tk_setPalette

gitk: do not invoke tk_setPalette

gitk uses themed widgets with a user selected theme, but also invokes
tk_setPalette to configure colors for the non-themed widgets including
the menubar. However, themes in general are expected to configure
those colors already. The builtin themes (default, alt, clam, classic on
unix/X11) all have compatible colors, and need no such reconfiguration,
and (most, if not all) available themes set the options database for this
purpose as well. Furthermore, gitk in the past avoided invoking
tk_setPalette on Windows to avoid some issues.

So, let's stop calling tk_setPalette everywhere, and just rely upon the
selected theme (possibly user installed) to have set all needed colors.

Note: if a user installs more than one theme using $themeloader, the last
one installed will have defined the colors to be used. Those colors will
probably be incorrect for any other set, including Tk's builtin set.

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