(struct gl_state_s): Delete `offset` field

(struct gl_state_s): Delete `offset` field

`gl_state` had an `offset` field because:

    For buffers, regex-emacs.c passes arguments to the
    UPDATE_SYNTAX_TABLE functions which are relative to BEGV

but the reality is that these arguments are byte offsets relative to
BEGV_BYTE whereas `offset` was counted in chars, so the two didn't
cancel each other out.

* src/syntax.h (struct gl_state_s): Delete `offset` field.
(UPDATE_SYNTAX_TABLE_FORWARD, UPDATE_SYNTAX_TABLE_BACKWARD)
(SYNTAX_TABLE_BYTE_TO_CHAR):
* src/syntax.c (SETUP_SYNTAX_TABLE, SETUP_SYNTAX_TABLE_FOR_OBJECT)
(update_syntax_table, skip_syntaxes): Simplify accordingly.