Remove unused NCL plot routines

[mattiarighi]

Some of the plotting routines in esmvaltool/diag_script/shared/plot/*.ncl are currently not used and will never be used, because the diagnostics using them have already been or will be ported in a different language (like the monsoon diagnostics). It would be good to remove them before the next release.

These include:

• GO_panels.ncl: the NCL diagnostics using them have been replaced by @ledm python diagnostics.
• misc_function.ncl: these are even remnants from the CCMValTool (!), in v1 they were used for the eyring06jgr and eyring13jgr. @hb326 are these still needed? They could be moved to aux_plotting.ncl.
• mjo_level1.ncl and mjo_level2.ncl: used in various mjo scripts in the mjo namelists. This was done by SMHI in v1, so I guess at some point @zklaus or someone in his group would port them (hence probably python).
• monsoon_domain_panels.ncl and monsoon_panels.ncl: same as above, @zklaus any idea of who is gonna port this and whether this will be in python?
• vector_scalar_map_polar.ncl also used by the mjo scripts, see above.

Furhtermore taylor_diagram.ncl and taylor_diagram_less_hardcoded.ncl could probably be merged in a single file (@axel-lauer ?) and the routines in legends.ncl could become part of aux_plotting.ncl.

[zklaus]

I will port the monsoon diagnostics, and yes, they will be in python.

[mattiarighi]

Will it be also the case for the mjo recipes? They are available on the private repo only, at the moment.

[zklaus]

I guess so. I will be in touch with Shiyu, who still works here and worked on the original mjo diagnostics.

[hb326]

The functions and procedures in misc_function.ncl are indeed still needed in eyring06jgr and eyring13jgr, even in v2. It is fine to move them to aux_plotting.ncl.