Use a dedicated 'deprecated' mark instead of add '-deprecated' for each spec

Also rename `@cases.SpecificationSelector.requiredBySpecification("xxx")`
to `@cases.mark_specifications("xxx")` because it's shorter and looks
like pytest's own syntax

irctest/specifications.py

@@ -5,10 +5,8 @@ import enum
 class Specifications(enum.Enum):
     RFC1459 = "RFC1459"
     RFC2812 = "RFC2812"
-    RFCDeprecated = "RFC-deprecated"
     IRC301 = "IRCv3.1"
     IRC302 = "IRCv3.2"
-    IRC302Deprecated = "IRCv3.2-deprecated"
     Oragono = "Oragono"
     Multiline = "multiline"
     MessageTags = "message-tags"