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, 1 insertions, 1 deletions

diff --git a/object.c b/object.c
index 16dce71671..3aa7cd20d2 100644
--- a/object.c
+++ b/object.c
@@ -1687,7 +1687,7 @@ rb_true(VALUE obj)
  */
 
 
-MJIT_FUNC_EXPORTED VALUE
+static VALUE
 rb_false(VALUE obj)
 {
     return Qfalse;