Revert regexp reentrancy abort patch

Revert regexp reentrancy abort patch

Although the patch does fix Bug#21688 and prevents a core dump,
it also makes the message-mode-propertize test fail; see:
http://lists.gnu.org/archive/html/emacs-devel/2015-11/msg01667.html
Perhaps someone else can come up with a better fix some day.
* src/syntax.c (update_syntax_table_forward):
Propertize even when truncated.
* src/syntax.h (UPDATE_SYNTAX_TABLE_FORWARD_FAST):
(UPDATE_SYNTAX_TABLE_FAST): Remove.
All callers changed back to the non-_FAST versions.