[ABNF] Remove keywords 'true' and 'false' again.

These were removed in a previous commit, because they are already classified as
(boolean) literals, but they were accidentally re-introduced, presumably due to
the type alias RFC referencing the previous version of the keyword grammar rule.

grammar/abnf-grammar.txt

@@ -395,7 +395,6 @@ keyword = %s"address"
         / %s"console"
         / %s"const"
         / %s"else"
-        / %s"false"
         / %s"field"
         / %s"for"
         / %s"function"
@@ -416,7 +415,6 @@ keyword = %s"address"
         / %s"self"
         / %s"static"
         / %s"string"
-        / %s"true"
         / %s"type"
         / %s"u8"
         / %s"u16"