Exclude abstract defs from "no overload matches" errors

[HertzDevil]

Abstract defs are never callable and therefore do not affect whether any overload matches a given call. (Even if they do, every abstract def must be subsumed by another non-abstract def.) Given:

module Foo
  abstract def foo(x : Int32)
end

class Bar
  include Foo

  def foo(x : Int32); end
end

Bar.new.foo('a')

# Error: no overload matches 'Bar#foo' with type Char
#
# Overloads are:
#  - Bar#foo(x : Int32)
#  - Foo#foo(x : Int32)

This PR removes Foo#foo(x : Int32) from the error message.

[sdogruyol]

Thank you @HertzDevil 🙏