Remove timestamp based rebuilds

Created by: ollieparanoid

Discussion leading up to this PR: #1167 (closed)

If you want to build a package without changing the version number, please use --force from now on. For example:

pmbootstrap build --force hello-world

Prior to this commit, changes were detected automatically (timestamp based rebuilds). However, that feature does not work as expected with the binary package repository we have now, and depending on how you use git, it has never worked. Close #1167 (closed), close #1156 (closed), close #1023 (closed) and close #985 (closed).