Remove override printf define from c receipe

This define would serve to use the integer-only version of printf rather
than the default. In commit 46470a9 (Remove override printf define from
cpp receipe), this was removed from the cpp recipe, probably because
this define could cause problems in C++ code and because the Due has
plenty of memory for this.

However, this same override was still present in the c recipe, so this
introduced an inconsistency between .c and .cpp files, where they would
end up getting different versions of printf. This commit fixes this by
also removing the override for .c files.

platform.txt

@@ -19,7 +19,7 @@ compiler.warning_flags.all=-Wall -Wextra
 
 compiler.path={runtime.tools.arm-none-eabi-gcc-4.8.3-2014q1.path}/bin/
 compiler.c.cmd=arm-none-eabi-gcc
-compiler.c.flags=-c -g -Os {compiler.warning_flags} -std=gnu11 -ffunction-sections -fdata-sections -nostdlib --param max-inline-insns-single=500 -Dprintf=iprintf -MMD
+compiler.c.flags=-c -g -Os {compiler.warning_flags} -std=gnu11 -ffunction-sections -fdata-sections -nostdlib --param max-inline-insns-single=500 -MMD
 compiler.c.elf.cmd=arm-none-eabi-g++
 compiler.c.elf.flags=-Os -Wl,--gc-sections
 compiler.S.cmd=arm-none-eabi-gcc