Revert "ClearCase: Check if ClearCase command is executable"

It introduced a warning:

SOFT ASSERT: "QFileInfo(m_settings.ccBinaryPath).isExecutable()" in file .\clearcaseplugin.cpp, line 382

on machines that do not have clearcase installed (which is the
majority).

This reverts commit d3bef1b0.

Change-Id: I7c5c599a9b2af175fd4d4c8d81b5c59dcbdffdba
Reviewed-by: Knut Petter Svendsen <knutpett@pvv.org>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>