Don’t warn about integer conversion in pdumper.c

Don’t warn about integer conversion in pdumper.c

Problem reported by Juanma Barranquero in:
https://lists.gnu.org/r/emacs-devel/2020-08/msg00279.html
and a similar glitch was reported by Eli Zaretskii (Bug#36597#67).
* src/pdumper.c: Remove -Wconversion pragma.
(ALLOW_IMPLICIT_CONVERSION, DISALLOW_IMPLICIT_CONVERSION):
Remove.  All uses removed.  Although -Wconversion may have
been helpful when writing pdumper.c it is now causing more
trouble than it’s worth here (just as in the rest of Emacs).
(dump_read_all): Avoid no-longer-necessary use of ‘size_t’
rather than ‘int’.