Remove application directory from QCoreApplication::libraryPaths()

Summary:
The directory containing the main application (read through argv[0]) is
by default added to the list of paths containing libraries.
This causes various methods to iterate through all entries of the plasmashell
install location, usually /usr/bin.
By explicitly removing the path, those unnecessary lookups can be avoided,
resulting in around 100ms quicker startup on a system with 4000 entries in
/usr/bin.

Test Plan:
Ran plasmashell with and without this fix, no changes except for a slightly
quicker startup and much less strace noise.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7255