Remove needless removal of trailing whitespace in check_code_state

[tompng]

IRB unnecessarily removes trailing white space before checking should_continue?. It results in failure of executing tricky syntax.

irb(main):001:0" <<"a "
irb(main):002:0" heredoc
irb(main):003:0> a (←whitespace here)
irb(main):004:0> 
irb(main):005:0> 

irb(main):001:0> % ruby (←whitespace here)
irb(main):002:0> 
irb(main):003:0> 
irb(main):004:0> 

irb(main):001:0* %
irb(main):002:0> 
irb(main):003:0> 

Reason why there was gsub and why it's not needed now

The old implementation of should_continue? (process_continue) was assuming tokens[-1] is newline and tokens[-2] is the last non-whitespace token. code.gsub(/\s*\z/, '').concat("\n") was ensuring that assumption.
https://github.com/ruby/irb/pull/611/files#diff-612b926e42ed78aed1a889ac1944f7d22229b3a489cc08f837a7f75eca3d3399L250-L262
The implementation is changed to skip continuous trailing newline, space, and comment-like tokens so we don't need that gsub anymore.