bitbake: toaster: Fix bitbake flexible path error from Bugzilla #12891

Commit 15340edce23e63b060c75114d508e1f76757239c introduced a way which
allowed bitbake to be found from the PATH variable when directory
structures different from poky are used. This just drops a leftover
bitbake definition which made the earlier implementation meaningless
by reassigning the path relative to oe-core/meta.

[YOCTO #12942]

(Bitbake rev: 99321236c516277c9d423b1b355bbce1245d9923)

(From poky rev: 161eaa28ed16f93d57f3d1c4be84f894e99ab72e)

Change-Id: Ida33083b2054d5b402a1d04cfcd021e8f9802707
Signed-off-by: Awais Belal <awais_belal@mentor.com>
Signed-off-by: David Reyna <David.Reyna@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>