Revert "Add a specialized instruction for `.nil?` calls"

This reverts commit 9faef3113fb4331524b81ba73005ba13fa0ef6c6.

It seemed to cause a failure on macOS Mojave, though I'm unsure how.
https://rubyci.org/logs/rubyci.s3.amazonaws.com/osx1014/ruby-master/log/20190802T034503Z.fail.html.gz

This tentative revert is to check if the issue is actually caused by the
change or not.

1 files changed, 0 insertions, 1 deletions

diff --git a/vm.c b/vm.c
index b4e2e3c5e6..115bdca503 100644
--- a/vm.c
+++ b/vm.c
@@ -1656,7 +1656,6 @@ vm_init_redefined_flag(void)
     OP(Call, CALL), (C(Proc));
     OP(And, AND), (C(Integer));
     OP(Or, OR), (C(Integer));
-    OP(NilP, NIL_P), (C(NilClass));
 #undef C
 #undef OP
 }