Remove `TOOLSUFFIX`.
Remove `TOOLSUFFIX`.

This is not set by any Kconfig variable, so is not currently available for
users anyway. Additionally some other Makefiles and fragments directly refer to
the compiler as `${TOOLPREFIX}gcc` so setting `TOOLSUFFIX` will not actually
result in using the indicated compiler consistently. This includes the kernel's
Makefile.

seL4/common-tool,winksaville/common-tool,hendersa/px4-common-tool,winksaville/common-tool,hendersa/px4-common-tool,seL4/common-tool,seL4/common-tool,hendersa/px4-common-tool,winksaville/common-tool