Deletes the cache files written under scopus_cache_dir(). A cache you
created in a directory of your own is left untouched.
Examples
# The managed cache is pointed at a temporary directory first, so that
# running this example cannot delete the checkpoints of a real harvest.
# Nothing is cached there, and clearing an absent cache is safe.
old <- Sys.getenv("R_USER_CACHE_DIR", unset = NA)
Sys.setenv(R_USER_CACHE_DIR = tempfile())
scopus_cache_clear()
if (is.na(old)) Sys.unsetenv("R_USER_CACHE_DIR") else
Sys.setenv(R_USER_CACHE_DIR = old)
