Remove unneeded SDK files from libpico/etc. (#2733)

Pico-SDK changed the output extension of their compilation from "x.c.obj"
to "x.c.o" meaning the removed Newlib and STDIO SDK wrappers were still
being linked in certain situations.  Update make-libpico Cmakefile to
remove the new names.

Fixes debug `printf` output.

Author: earlephilhower