libc/newlib: Remove duplicate declaration of z_malloc_partition

The newlib stanza in libc-hooks.h declares z_malloc_partition, but also
sets Z_MALLOC_PARTITION_EXISTS. That preprocessor symbol controls
whether z_malloc_partition is declared later in the file, causing
a duplicate declaration. Remove the newlib-specific one as redundant.

Signed-off-by: Keith Packard <keithp@keithp.com>