Don't force TLS on behalf of heap profiling.

Revert 6716aa8352 (Force use of TLS if
heap profiling is enabled.).  No existing tests indicate that this is
necessary, nor does code inspection uncover any potential issues.  Most
likely the original commit covered up a bug related to tsd-internal
allocation that has since been fixed.

configure.ac

@@ -793,11 +793,6 @@ fi
 AC_MSG_CHECKING([configured backtracing method])
 AC_MSG_RESULT([$backtrace_method])
 if test "x$enable_prof" = "x1" ; then
-  if test "x${force_tls}" = "x0" ; then
-    AC_MSG_ERROR([Heap profiling requires TLS]);
-  fi
-  force_tls="1"
-
   if test "x$abi" != "xpecoff"; then
     dnl Heap profiling uses the log(3) function.
     LIBS="$LIBS -lm"