libdecnumber: Eliminate redundant declarations

Eliminate redundant declarations of symbols DPD2BIN and BIN2DPD in
various .c source files.  These symbols are already declared in decDPD.h and
thus will trigger 'redundant redeclaration of ?XXX?' warnings, which, of
course, may fail QEMU compilation.

Signed-off-by: Tom Musta <tommusta@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>