Revert "tests: Use explicit coverage tracking"

This reverts commit b00e886142.

This fixes reporting of coverage by the few tests that run outside of
vim.

The commit that is being reverted doesn't explain why it was committed,
and the git history suggests it was reverted once and then reintroduced
later again. None of those commits explains the why. I can only guess
that the last time this was committed was an attempt to fix coverage
reporting to outside of docker, which it didn't, and additionally it
made coverage gathering less robust (see previous commit). So this is
yet another fix for the inaccurate coverage reporting.

Or maybe it was because `--cov` without `=taskwiki` an argument reports
coverage for tests instead of taskwiki code? Nevermind, I guess, now it
works well.

tests/base.py

@@ -70,7 +70,6 @@ class IntegrationTest(object):
         self.command('let g:taskwiki_data_location="{0}"'.format(self.dir))
         self.command('let g:taskwiki_taskrc_location="{0}"'.format(self.taskrc_path))
         self.command('let g:vimwiki_list = [{"syntax": "default", "ext": ".txt","path": "%s"}]' % self.dir)
-        self.command('let g:taskwiki_measure_coverage="yes"')
         self.command('let g:taskwiki_markup_syntax="{0}"'.format(self.markup))
 
     def setup(self):