git-commit-mode.el: remove 'saveplace dependency

The only reason we required 'saveplace was to get rid of a
byte-compilation warning (assignment to free variable `save-place'),
i.e. we don't actually use any of its functionality.

Declaring `save-place' ourselves is sufficient for those purposes.

While we're at it, let's save a function call by just checking whether
`save-place' is non-nil instead of whether `toggle-save-place' has a
function definition.

See commits f5b36db4 and 830b538b.

Signed-off-by: Pieter Praet <pieter@praet.org>