pppd: Don't free static string

pppd: Don't free static string

Commit fcb076c2 ("Various fixes for errors found by coverity static
analysis (#109)", 2019-05-06) added statements to free the result
returned from get_first_ethernet().  However, the result of
get_first_ethernet() is not dynamically allocated, either on Linux
or Solaris.  Hence this removes the unnecessary (and dangerous)
free() statements.

Fixes: fcb076c2 ("Various fixes for errors found by coverity static analysis (#109)")
Reported-by: Florian Kohnhäuser <florian@kohnhaeuser.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>