Tidy up SVN file properties

[DJPaul]

We need to tidy up SVN properties for all our files. We have svn:executable incorrectly set, as well as svn:mime-type.
The latter is causing Trac to not syntax highlight affected files, e.g. ​https://buddypress.trac.wordpress.org/browser/trunk/src/bp-core/bp-core-template-loader.php

See ​https://wordpress.slack.com/archives/buddypress/p1425691464000639 for discussion where this was noticed.

[djpaul]

In 9604:

Remove unwanted svn:executable and svn:mime-type SVN properties throughout project.

In particular, incorrect use of svn:mime-type on PHP script files is causing Trac to not provide syntax highlighting when browsing a file.
e.g. ​https://buddypress.trac.wordpress.org/browser/trunk/src/bp-core/bp-core-template-loader.php

Fixes #6283

[DJPaul]

For the convenience of similar future tidy up, I used these sorts of commands to do this:

svn propdel svn:executable -R
find . \( -name "*.php" -print -name "*.js" -print -name "*.css" -print \) | xargs svn propdel svn:mime-type