sstate.bbclass: Only remove sstate file when task is existed

sstate.bbclass: Only remove sstate file when task is existed

This can improve the performance a lot for "bitbake <recipe-native/cross/crosssdk>
-ccleansstate" when there are a lot of sstate files.

For example:
* Before
  $ bitbake quilt-native -ccleansstate
  - Check log.do_cleansstate:
  Removing /sstate-cache/*/*/sstate:quilt-native:x86_64-linux:0.65:r0:x86_64:3:*_package.tgz*
  Removing /sstate-cache/*/*/sstate:quilt-native:x86_64-linux:0.65:r0:x86_64:3:*_package_qa.tgz*
  Removing /sstate-cache/*/*/sstate:quilt-native:x86_64-linux:0.65:r0:x86_64:3:*_package_write_rpm.tgz*
  Removing /sstate-cache/*/*/sstate:quilt-native:x86_64-linux:0.65:r0:x86_64:3:*_packagedata.tgz*
  Removing /sstate-cache/*/sstate:quilt-native::0.65:r0::3:*_populate_lic.tgz*
  Removing /sstate-cache/*/*/sstate:quilt-native:x86_64-linux:0.65:r0:x86_64:3:*_populate_sysroot.tgz*

  There are no package tasks for quilt-native, so the first 4 lines doesn't
  make any sense, but the glob pattern "sstate-cache/*/*" is very time
  consuming when there are no disk caches. E.g., I have more than 600,000
  sstate files:
  - Without disk caches
  # echo 3 >/proc/sys/vm/drop_caches
  $ time python3 -c 'import glob; glob.glob("/sstate-cache/*/*/sstate:quilt-native:x86_64-linux:0.65:r0:x86_64:3:*_package.tgz*")'
    real    4m32.583s
    user    0m5.768s
    sys     0m12.892s

  - With disk caches (e.g., run it in the second time)
  $ time python3 -c 'import glob; glob.glob("/sstate-cache/*/*/sstate:quilt-native:x86_64-linux:0.65:r0:x86_64:3:*_package.tgz*")'
    real    0m5.128s
    user    0m2.772s
    sys     0m2.308s

  So the 4 removing *package* commands cost more than 20s or 272s in theory.

* After
  $ bitbake quilt-native -ccleansstate
  - Check log.do_cleansstate:
  Removing /sstate-cache/*/sstate:quilt-native::0.65:r0::3:*_populate_lic.tgz*
  Removing /sstate-cache/*/*/sstate:quilt-native:x86_64-linux:0.65:r0:x86_64:3:*_populate_sysroot.tgz*

  We can see that it saved 20s or 272s in theory.

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