remove data_src parameter in epmc_ftxt()

[njahn82]

From the onboarding review:

epmc_ftxt() includes a data_src = "pmc" argument, but the function is hard-coded to always use PMC as a source--this argument is only used to throw an error if anything other than PMC is used. Perhaps better to remove this argument and clarify in documentation, as using `args(epmc_ftxt)`` might sugget to a user that they can use another source.