Remove drug end date convention

[MaximMoinat]

As noted by @MelaniePhilofsky: the drug end date convention is now on Themis, and there is no need to repeat it here.

• Shorten paragraph on drug end date from v5.3 and v5.4 and v6.0 field level csv
• Render html.

[MaximMoinat]

@clairblacketer When I execute rmarkdown::render_site(), it completes but the tables with field description do not render in the cdm53 and cdm54 documentation. Could you try?

[MaximMoinat]

@clairblacketer I was able to render the pages using the 'knit' button directly in RStudio.

As discussed in Yesterday's meeting, this is just a small start for a bigger documentation refactoring. Other descriptions should also be shortened (e.g. in drug exposure the drug_concept_id, quantity and days_supply fields are still full paragraphs of convention description which might be covered by our new Themis library).