libev: remove all previous libev related code

except:
 * cmake integration (LWS_USE_LIBEV, LWS_WITH_LIBEV)
 * text in changelog and READMEs
 * server option (LWS_SERVER_OPTION_LIBEV)
 * code for printing the ev_backend() -- but kept disabled

Signed-off-by: Alejandro Mery <amery@geeks.cl>