Fix script local function is not found by removing SID hack

[rhysd]

Running :Unite outline caused the following error:

[unite.vim] function neobundle#autoload#_command[14]..unite#helper#call_unite[33]..unite#start[1]..unite#start#standard[50]..unite#candidates#_recache[74]..<SN
R>158_recache_candidates_loop[42]..<SNR>158_get_source_candidates[40]..37[1]..unite#get_all_sources[1]..unite#variables#all_sources[2]..unite#init#_sources[11]
..unite#init#_default_scripts[54]..script /Users/rhysd/.vim/bundle/unite-outline/autoload/unite/sources/outline.vim[506]..function unite#sources#outline#import
[2]..script /Users/rhysd/.vim/bundle/unite-outline/autoload/unite/sources/outline/modules/file_cache.vim[39]..function unite#sources#outline#import[2]..script
/Users/rhysd/.vim/bundle/unite-outline/autoload/unite/sources/outline/modules/util.vim[44]../Users/rhysd/.vim/bundle/unite-outline/autoload/unite/sources/outli
ne/modules/base.vim, line 53
[unite.vim] Vim(let):E700: Unknown function: <SNR>158_Module_bind
[unite.vim] Error occurred in source initialization!
[unite.vim] Source name is /Users/rhysd/.vim/bundle/unite-outline/autoload/unite/sources/outline.vim

Since function() can now take s: prefix for function name as first argument, the s:SID hack is no longer necessary. This PR removes the hack and I confirmed the error was fixed by the modification.

[rhysd]

If you prefer making this plugin compatible with older Vim which does not support s: in function(), please let me know. I'll update this PR not to break compatibility.