Do not provide a default value for `module-link`

The old default value was an abomination which never should have been
allowed to see the light of day.

This patch removes the default, which is a backwards incompatible change
with low probability of causing anyone any real trouble (a repo with
submodules, displayed by cgit using the default value of `module-link`,
is very unlikely to actually generate working links).

Signed-off-by: Lars Hjemli <hjemli@gmail.com>