Revert "toolchain/toolchain-external: error if BR2_TOOLCHAIN_EXTERNAL_PATH is not set"

This reverts commit 8e91385a2cb81c5f8114354f4494230e2aebb93a.

This commit is incorrect, as it is perfectly valid for
BR2_TOOLCHAIN_EXTERNAL_PATH to be empty. The help text of
BR2_TOOLCHAIN_EXTERNAL_PATH even documents it as a supported case:

          If empty, the compiler will be searched in $PATH.

Commit 392b0a26f5fd8d3aedce7e0c15f9762a79312f01 ("toolchain-external:
default BR2_TOOLCHAIN_EXTERNAL_PATH to empty") even made that the
default saying "In addition, it in fact works correctly when it is
empty. In that case, the toolchain will be searched in PATH."

A user has reported that commit
8945ba49480a7885d104445ca855ef7681a374ff (the backport of 8e91385a2c to
the 2022.02.x LTS branch) breaks his use-case:

  https://lore.kernel.org/buildroot/CADBnMvhgaozAgZgy3njckjL1i0U6bZ0fLrq-kdFF-qpGhFWgmw@mail.gmail.com/

Reported-by: Kristof Havasi <havasiefr@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[yann.morin.1998@free.fr: reference 8e91385a2c on master]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

toolchain/toolchain-external/pkg-toolchain-external.mk

@@ -69,12 +69,7 @@ ifeq ($(BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD),y)
 TOOLCHAIN_EXTERNAL_INSTALL_DIR = $(TOOLCHAIN_EXTERNAL_DOWNLOAD_INSTALL_DIR)
 else
 TOOLCHAIN_EXTERNAL_INSTALL_DIR = $(abspath $(call qstrip,$(BR2_TOOLCHAIN_EXTERNAL_PATH)))
-ifeq ($(BR2_TOOLCHAIN_EXTERNAL)$(BR_BUILDING),yy)
-ifeq ($(TOOLCHAIN_EXTERNAL_INSTALL_DIR),)
-$(error No external toolchain path set, check your BR2_TOOLCHAIN_EXTERNAL_PATH setting)
-endif
 endif
-endif # BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD
 
 ifeq ($(TOOLCHAIN_EXTERNAL_INSTALL_DIR),)
 ifneq ($(TOOLCHAIN_EXTERNAL_PREFIX),)