Backout replacing manually crafted hex regexes with [:xdigit:].

Backout replacing manually crafted hex regexes with [:xdigit:].

* lisp/progmodes/verilog-mode.el (verilog-delay-re):
(verilog-type-font-keywords, verilog-read-always-signals-recurse):
(verilog-is-number): Backout replacing manually crafted hex regexes with
[:xdigit:] (Bug#36167).  This repairs Verilog-mode regressions;
Verilog-mode maintains back-compatibility with Emacsen before this syntax
was supported.