[flang][OpenMP] Delete definitions of non-delimited end-directives, NFC (#205932)
Delimited directives are those that come in begin/end pairs, e.g. "begin
declare target"/"end declare target". Other block-associated directives
in Fortran do have end-forms, but they don't need to have specific
directive enums. Some such enums have been used in the past, but are not
anymore. Delete those extraneous definitions to clean up the OMP.td
file.