Revert "when version.txt and internal version differ, use the internal one"

Revert "when version.txt and internal version differ, use the internal one"

netdata should compare the saved version number with its repository,
because comparing the Git ID for the "ab-debian" branch makes no sense:
these commits aren't available in the official repository at all, and
therefore the update check would always trigger and never find the
correct changes on GitHub ...

This reverts commit 7647a12b03d31e90cc2edd1546132bc0be973766.