env: Drop env_get_char_spec() and old, unused .get_char() implementations

Commit b2cdef48 ("env: restore old env_get_char() behaviour")
dropped the .get_char() method from struct env_driver, but left the two
existing implementations (eeprom and nvram) in case someone would use
them by overwriting weak function env_get_char_spec().

Since this was never done in the 3.5 years, let's drop these methods and
simplify the code.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>