src/poky:flex: build occasional build failures in test suite compile

GNU Make 4.4 has added --shuffle, which randomly orders build jobs and
is great at exposing missing dependencies. Using this the known problem
with the flex test suite build was found and resolved, so remove the
workaround of doing a non-parallel build and apply a patch.

Also remove redundant Make variables that shouldn't be needed to build
the test suite.

(From OE-Core rev: 1bc37e10690dc906da41b9592bc8776bddf4b90b)

Signed-off-by: Ross Burton <ross.burtonarm.com>
Signed-off-by: Richard Purdie <richard.purdielinuxfoundation.org>

Author: rossburton