evolve: remove not required calls to phases.retractboundary()
To make sure phases are preserved during evolve, we started using the
phases.new-commit config option. Now we don't need to call
phases.retractboundary(). Let's remove the redundant code.
#require test-repo pyflakes
Copied from Mercurial core (60ee2593a270)
$ cd "`dirname "$TESTDIR"`"
run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)
$ hg locate 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
> | xargs pyflakes 2>/dev/null