The `periph_flash_common` feature was only defined here to trigger
inclusion of a source file with common functions.
It even only defines private symbols `_lock` and `_unlock` so no reason
to expose it to the build system.
And in practice, all stm cpus providing `periph_flashpage` or
`periph_eeprom` were required to provide `periph_flash_common` to allow
including it.

The previous implementation was only parsing in the modules were in
`FEATURES_REQUIRED` wich did not take cases of `FEATURES_OPTIONAL` into
account.
And also, in the same time, as the dependencies was declared in
`Makefile.include` it was processed before `Makefile.dep` so never handled
cases where a module could depend on `periph_flashpage` or
`periph_eeprom` feature.

It is replaced by selecting the common source file when module using it
are included.

The now useless feature `periph_flash_common` is removed from
`FEATURES_PROVIDED`.