Please delete 4.2.0 branch

[ericonr]

The branch was merged in #158

Its existence makes it impossible to download a tarball with the URL https://github.com/epics-base/p4p/archive/4.2.0.tar.gz, which instead simply returns the following text: the given path has multiple possibilities: #<Git::Ref:0x00007f82c2f2eb20>, #<Git::Ref:0x00007f82c2f2d680>

It is of course possible to specify the tag with refs/tags/, but there's no reason to require it :)

Reference: https://www.drupal.org/project/webform/issues/3264249

[mdavidsaver]

refs/heads/4.2.0 has been removed. @ericonr thanks for pointing this out.