Remove ConfigService checking for Python and Matlab executable names

[martyngigg]

The ConfigService behaves slightly differently if the executable name is Python or Matlab. At the time this was due to the fact that we needed to know where the Mantid bin directory was if running in a standalone interpeter in a different directory.

Now that we have the MANTIDPATH variable this behaviour should be changed so that Mantid is consistent regardless of the executable name.

The change will mean that when the ConfigService starts it looks for the Mantid.properties file in the following way:
(a) If MANTIDPATH is defined then use this path, else
(b) use the executable directory.

[martyngigg]

This issue was originally trac ticket 2256