Migrate functionality from private _parse_bsrn and _parse_epw to their read functions

[AdamRJensen]

During the deprecation of the parse_bsrn and parse_epw functions (#2466 and #2467), the parsing functionality was left in the corresponding private functions. This was done to quickly deprecate the parse functions as discussed in #2444.

For the sake of simplicity, the functionality of these private functions can be moved to the corresponding read functions, and the private functions can be eliminated. This avoids duplicate documentation and reduces the number of functions, and is more in line with other iotools.