Remove x_to_nwb conversion functionality from the repo

[sgratiy]

Describe the use case that is addressed by this feature.
The x_to_nwb conversion has been moved to a dedicated repo https://github.com/byte-physics/x-to-nwb
Thus we should drop x_to_nwb from ipfx

Describe the solution you'd like

• Contact Thomas Braun the maintainer of the https://github.com/byte-physics/x-to-nwb to coordinate with him our plan, also Brian Kalmbach and Thomas Chartrand who work with Human data collaborators who convert .abf and .dat files to NWB2.
• Remove the ipfx/x_to_nwb and all of its tests
• Check that documentation does not include references to the removed functionality
• Add in documentation the link to the dedicated repo https://github.com/byte-physics/x-to-nwb

Validation criteria:

• ipfx tests pass

[tmchartrand]

One thought with this change - it may be worth also moving the nwb_to_pdf functionality as well. IPFX has it's own plotting functionality, and it seems like the nwb_to_pdf is primarily useful as a dump of the raw sweeps to validate the conversion (both by visual inspection, and some special functionality for checking the stimulus reconstruction against actual stimulus recordings for converted abf files that contained paired recordings).