multilib.bbclass: deltask populate_sdk and populate_sdk_ext

The "bitbake image -cpopulate_sdk/ext" generates SDK/eSDK for all multilib
variants, so "bitbake lib32-image -cpopulate_sdk/ext" is not needed, and it
doesn't work well, for example:

MACHINE ?= "qemux86-64"
require conf/multilib.conf
MULTILIBS = "multilib:lib32"
DEFAULTTUNE_virtclass-multilib-lib32 = "x86"

$ bitbake lib32-core-image-minimal -cpopulate_sdk_ext
[snip]
Exception: FileExistsError: [Errno 17] File exists: '/buildarea/lyang1/test_q64/tmp/sysroots-components/core2-64/openssl/sysroot-providers/openssl10' -> '/buildarea/lyang1/test_q64/tmp/work/qemux86_64-pokymllib32-linux/lib32-core-image-minimal/1.0-r0/lib32-recipe-sysroot/sysroot-providers/openssl10'
[snip]

The problem is populate_sdk_ext installs all multilib variants, and
extend_recipe_sysroot() handles foo-image depends lib32-foo-image, but doesn't
handle lib32-foo-image depends foo-image, we can use a lot of trick ways to make
it work:
1) Get foo-image's RECIPE_SYSROOT when build lib32-foo-image
2) Handle conflicts with foo-image.do_rootfs
3) Handle conflicts when "bitbake lib32-foo-image foo-image -cpopulate_sdk_ext"

And maybe other potential problems, this looks painful, so just delete the task.

[YOCTO #12210]

(From OE-Core rev: 77144bc808be02deb3351c9c1bf5b4f2b8c3a6ec)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/multilib.bbclass

@@ -99,6 +99,8 @@ python __anonymous () {
         d.setVar("LINGUAS_INSTALL", "")
         # FIXME, we need to map this to something, not delete it!
         d.setVar("PACKAGE_INSTALL_ATTEMPTONLY", "")
+        bb.build.deltask('do_populate_sdk', d)
+        bb.build.deltask('do_populate_sdk_ext', d)
         return
 
     clsextend.map_depends_variable("DEPENDS")