Remove PHPStorm specific exclude from .gitignore

As discussed in https://github.com/SURFnet/saml2/commit/c5d13a48a2033a293ce7005809d70f15b8703b11#commitcomment-7902334
Merge pull request #5 from SURFnet/improvement/remove-ide-specific-excludes-from-gitignore

Remove PHPStorm specific exclude from .gitignore