Remove parse_multipart()

[yihui]

since shiny no longer uses it: https://github.com/rstudio/shiny/issues/3074.

[yihui]

Unfortunately, it can't be removed since it has been used by more packages:

• https://github.com/MazamaScience/beakr/blob/e1554654abd73a755bacd655e223a28e734db725/R/utils-internal.R#L83
• https://github.com/JohnCoene/ambiorix/blob/c264505c9d88a3fba278b3208557ea752c4302de/R/parser.R#L19