Revert of Internalize strings in-place

src/regexp/regexp-macro-assembler.cc

Index: src/regexp/regexp-macro-assembler.cc
diff --git a/src/regexp/regexp-macro-assembler.cc b/src/regexp/regexp-macro-assembler.cc
index 73797e4416b8b198e3c13df50b56587bd6c04fa3..0a7f5c1b9e295727ac672cc008adee0dbfc41ed6 100644
--- a/src/regexp/regexp-macro-assembler.cc
+++ b/src/regexp/regexp-macro-assembler.cc
@@ -133,8 +133,6 @@
   } else if (subject->IsSlicedString()) {
     start_index += SlicedString::cast(subject)->offset();
     subject = SlicedString::cast(subject)->parent();
-  } else if (subject->IsThinString()) {
-    subject = ThinString::cast(subject)->actual();
   }
   DCHECK(start_index >= 0);
   DCHECK(start_index <= subject->length());
@@ -241,9 +239,6 @@
     subject_ptr = slice->parent();
     slice_offset = slice->offset();
   }
-  if (StringShape(subject_ptr).IsThin()) {
-    subject_ptr = ThinString::cast(subject_ptr)->actual();
-  }
   // Ensure that an underlying string has the same representation.
   bool is_one_byte = subject_ptr->IsOneByteRepresentation();
   DCHECK(subject_ptr->IsExternalString() || subject_ptr->IsSeqString());