Remove unneeded output size parameter from internal count_matches utility

[davidwendt]

Description

Removes output_size parameter from cudf::strings::detail::count_matches utility since the output size should equal the input size from the first parameter. This also removes an unnecessary assert() call. The parameter became unnecessary as part of the large strings work.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[davidwendt]

/merge