Remove IssueSeverity support from StatusCode and release

The IssueSeverity support in StatusCode was removed after discussion at the Gaudi workshop. Except for one place in ATLAS this was not being used and introduced unnecessary overhead (both in time and memory) to the StatusCode object.

After discussion it was decided to completely remove IssueLogger, IssueSeverity and all associated source files and tests from Gaudi as nobody seems to be using it.

Notes:

• Due to the removal of the <memory> include in StatusCode, some missing includes were exposed in unrelated headers that are fixed as well.
• The larger StatusCode changes we discussed at the workshop is going to come as a separate MR.