Remove $(TOOLCHAIN_DIR)/bin and $(STAGING_DIR)/{usr/bin,bin} from the PATH

These shouldn't be needed. Even when the cross-compiler is in
$(STAGING_DIR)/usr/bin, we anyway use an absolute path for TARGET_CC,
TARGET_LD and al.

Not having $(STAGING_DIR)/{usr/bin,bin} in the PATH will avoid having
Buildroot trying to run target binaries.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>