mips: kill unused definitions in binfmt_elf[on]32.c

elf_caddr_t: unused since 2002
jiffies_to_timeval: unused since 2015
TASK_SIZE: used only downstream of SET_PERSONALITY2(), and after that
point the normal definition results in TASK_SIZE32 just fine.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>