(maint) Refactor shared example from parser/compiler_spec

Previously this spec's tests were a shared_example that had been shared
between runs with parser=current and parser=future. The future compiler
tests were moved out into
spec/integration/parser/future_compiler_spec.rb in 7a6a7d8. This maint
fix just removes the unused rspec construction of shared_examples_for
from around the specs in parser/compiler_spec, since they are only being
run against the current parser now.