Revert "of: net: support NVMEM cells with MAC in text format"

This reverts commit 9ed319e4

.

We can already post process a nvmem cell value in a particular driver.
Instead of having yet another place to convert the values, the post
processing hook of the nvmem provider should be used in this case.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>