erb.rb: drop unreachable method

This seems to be unreachable from first introduction at r21286.

In ERB implementation, `#empty?` is only called for each member of
return value of `String#scan`, and `ERB::Compiler::PercentLine` is never
returned from `String#scan`.

Also, in `ERB::Compiler#compile`, as it's yielded only when stag is nil,
methods called to `ERB::Compiler::PercentLine` object yielded from
`ERB::Compiler::TrimScanner#scan` are only: `#nil?`, `#==`, `to_s`.

Thus `ERB::Compiler::PercentLine#empty?` is never used.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60032 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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