Remove overlapping inputForm=LowForm tests

Remove tests from LoweringCompilerSpec testing the placement of
inputForm=LowForm legacy, custom transforms. This behavior is already
tested in the CustomTransformSpec.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>
(cherry picked from commit f5dd8ddc)

# Conflicts:
#	src/test/scala/firrtlTests/LoweringCompilersSpec.scala