git-instaweb: Remove pidfile after stopping web server

git-instaweb: Remove pidfile after stopping web server

This way running e.g. "git instaweb" after "git instaweb --stop" would
not try to kill already stopped web server.

This is probably important only for those web servers that are
"daemonized" by git-instaweb itself, i.e. for those where it is
git-instaweb that creates pidfile.  Currently it is includes only
'mongoose' web server, but it would also include 'plackup' web server
(added in later commit).

Signed-off-by: Jakub Narebski <redacted>
Acked-by: Petr Baudis <redacted>
Acked-by: Eric Wong <redacted>
Signed-off-by: Junio C Hamano <redacted>