Suppress compiler warnings in readfuncs.c.

Commit 7357558f introduced "(void) token;"
into the READ_TEMP_LOCALS() macro, to suppress complaints from gcc 4.6
when the value of token was not used anywhere in a particular node-read
function.  However, this just moved the warning around: inspection of
buildfarm results shows that some compilers are now complaining that token
is being read before it's set.  Revert the READ_TEMP_LOCALS() macro change
and instead put "(void) token;" into READ_NODE_FIELD(), which is the
principal culprit for cases where the warning might occur.  In principle we
might need the same in READ_BITMAPSET_FIELD() and/or READ_LOCATION_FIELD(),
but it seems unlikely that a node would consist only of such fields, so
I'll leave them alone for now.