doc: Remove description of experimental warnings related pattern matching from documentation

Ruby 3.2.0 has been released and all experimental warnings about pattern matching have been removed.
Experimental warnings about pattern matching are no longer output, so I remove description about it from documentation as well.

cf. https://bugs.ruby-lang.org/issues/18585
cf. db6b23c76cbc7888cd9a9912790c2068703afdd0
cf. https://twitter.com/k_tsj/status/1606956336037900289?s=20&t=-_PSYLhYPtYsB9FZhtXl5A

doc/syntax.rdoc

@@ -12,7 +12,7 @@ Assignment[rdoc-ref:syntax/assignment.rdoc] ::
   +if+, +unless+, +while+, +until+, +for+, +break+, +next+, +redo+
 
 {Pattern matching}[rdoc-ref:syntax/pattern_matching.rdoc] ::
-  Experimental structural pattern matching and variable binding syntax
+  Structural pattern matching and variable binding syntax
 
 Methods[rdoc-ref:syntax/methods.rdoc] ::
   Method and method argument syntax

doc/syntax/control_expressions.rdoc

@@ -255,7 +255,7 @@ Again, the +then+ and +else+ are optional.
 The result value of a +case+ expression is the last value executed in the
 expression.
 
-Since Ruby 2.7, +case+ expressions also provide a more powerful experimental
+Since Ruby 2.7, +case+ expressions also provide a more powerful
 pattern matching feature via the +in+ keyword:
 
   case {a: 1, b: 2, c: 3}

error.c

@@ -217,7 +217,6 @@ rb_warning_category_enabled_p(rb_warning_category_t category)
  *
  * +:experimental+ ::
  *   experimental features
- *   * Pattern matching
  *
  * +:performance+ ::
  *   performance hints