Avoid warnings with `ruby -cw`

```console
% for rb in `git ls-files '*/mrblib/*.rb' 'mrblib'`; do ruby30 -cw $rb > /dev/null; done
mrbgems/mruby-array-ext/mrblib/array.rb:389: warning: assigned but unused variable - ary
mrbgems/mruby-array-ext/mrblib/array.rb:663: warning: assigned but unused variable - len
mrbgems/mruby-hash-ext/mrblib/hash.rb:119: warning: possibly useless use of a variable in void context
mrbgems/mruby-hash-ext/mrblib/hash.rb:259: warning: assigned but unused variable - keys
mrbgems/mruby-io/mrblib/io.rb:229: warning: literal in condition
mrbgems/mruby-io/mrblib/io.rb:280: warning: literal in condition
mrbgems/mruby-string-ext/mrblib/string.rb:347: warning: assigned but unused variable - len
mrbgems/mruby-toplevel-ext/mrblib/toplevel.rb:2: warning: parentheses after method name is interpreted as an argument list, not a decomposed argument
```

1 files changed, 0 insertions, 2 deletions

diff --git a/mrbgems/mruby-array-ext/mrblib/array.rb b/mrbgems/mruby-array-ext/mrblib/array.rb
index 7fbb948d2..7520b932f 100644
--- a/mrbgems/mruby-array-ext/mrblib/array.rb
+++ b/mrbgems/mruby-array-ext/mrblib/array.rb
@@ -386,7 +386,6 @@ class Array
     end
 
     beg = len = 0
-    ary = []
     if block
       if arg0.nil? && arg1.nil? && arg2.nil?
         # ary.fill { |index| block }                    -> ary
@@ -660,7 +659,6 @@ class Array
     return to_enum :keep_if unless block
 
     idx = 0
-    len = self.size
     while idx < self.size do
       if block.call(self[idx])
         idx += 1