Revert "Object#in? also accepts multiple parameters"

This reverts commit ebf69ab1636df74c76332c53bcd3d8494fb91b45.

`in?` must not take multiple parameters because its behavior would be
ambiguous:

    # Test if "B" is included in a list of names with `"B".in?(*names)`:
    names = ["BMorearty"]
    "B".in?(*names)   # => true

    names = ["BMorearty","rubyduo"]
    "B".in?(*names)   # => false

Conflicts:
	activesupport/lib/active_support/core_ext/object/inclusion.rb
	activesupport/test/core_ext/object/inclusion_test.rb
Revert the guide after removing varargs from `in?`.
Merge pull request #10394 from BMorearty/remove-varargs-from-in

Remove varargs from `Object#in?`