cgit.c: Do not reset HOME after unsetting it.

The number of odd cases in which git will try to read config is far too
great to keep putting a bandaid over each one, so we'll just unset it.

If it turns out that scripts really liked to know about $HOME, we can
always reset it in the filter forks.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>