Rename unused variable

This commit removes below Ruby's warning.

* "warning: assigned but unused variable"
Remove discarding method definition

This commit removes below Ruby's warning.

* "warning: method redefined"
Merge pull request #265 from kachick/improve-remove_warnings

Clean up Ruby's warnings in lib/
Fix some YARD for API documents

Fix YARD for Money::Currency#symbol_first

Fix YARD for Money::Formatting#format

Fix YARD for Money::Formatting#normalize_formatting_rules

Fix YARD for Money::Parsing::ClassMethods#parse

Fix YARD for Money::Parsing::ClassMethods#from_string

Fix YARD for Money::Parsing::ClassMethods#from_numeric

Fix YARD for Money::Parsing::ClassMethods#from_fixnum

Fix YARD for Money::Parsing::ClassMethods#from_float

Fix YARD for Money::Parsing::ClassMethods#from_bigdecimal

Fix YARD for Money#initialize

Fix YARD for Money#allocate

Fix YARD for Money#split