encoding.c (enc_memsize): remove unnecessary function

Trims our binary size slightly (numbers from x86-64):

       text    data     bss     dec     hex filename
    2795184   22766   72024 2889974  2c18f6 ruby.before
    2795112   22766   72024 2889902  2c18ae ruby.after

[ruby-core:65304]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48255 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-11-04

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48256 b2dd03c8-39d4-4d8f-98ff-823fe69b080e