Revert "Fallback to stop-the-world compaction on kernel < 5.13 hosts"

This reverts commit aac790f8ffe107c24f2487e93ab6f5b5598c4419.

Reason for revert: Not working as intended. The userfaultfd GC is still being chosen even if kernel version is < 5.13 (b/242181443).
Test: presubmit

Change-Id: I65a05ed44539e5629f4b4f584823cca5e3ca36a9
diff --git a/runtime/gc/collector/mark_compact.cc b/runtime/gc/collector/mark_compact.cc
index 21e63e2..8240711 100644
--- a/runtime/gc/collector/mark_compact.cc
+++ b/runtime/gc/collector/mark_compact.cc
@@ -65,7 +65,7 @@
 // on target is not required as MREMAP_DONTUNMAP and userfaultfd were enabled
 // together.
 #ifdef ART_TARGET
-static constexpr bool gHaveMremapDontunmap = true;
+static const bool gHaveMremapDontunmap = true;
 #else
 static const bool gHaveMremapDontunmap = IsKernelVersionAtLeast(5, 13);
 #endif
@@ -74,12 +74,6 @@
 static bool ShouldUseUserfaultfd() {
 #if !defined(__linux__)
   return false;
-#elif !defined(ART_TARGET)
-  // TODO: Try removing the following constraint once the host kernel version is
-  // upgraded (b/242181443)
-  if (!gHaveMremapDontunmap) {
-    return false;
-  }
 #endif
   int fd = syscall(__NR_userfaultfd, O_CLOEXEC | UFFD_USER_MODE_ONLY);
 #ifndef ART_TARGET