Remove LoadSNSNexus algorithm re #7072

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Make LoadNexus use LoadTOFRawNexus instead of LoadSNSNexus re #7072

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>
Resolve merge re #7072