Test_openjdk8_hs_sanity.external_x86-64_linux is triggered twice a day

[sophia-guo]

https://ci.adoptopenjdk.net/view/Test_external/job/Test_openjdk8_hs_sanity.external_x86-64_linux/ is triggered twice daily by https://ci.adoptopenjdk.net/job/build-scripts/job/jobs/job/jdk8u/job/jdk8u-linux-x64-hotspot/ and
https://ci.adoptopenjdk.net/job/build-scripts/job/jobs/job/jdk8u/job/jdk8u-linux-x64-hotspot-jfr/, which will trigger all hotspot 8 external sanity tests job twice daily and test same docker images. Ideally test jobs should be triggered by docker images updates #773

Currently we only have limited resources for external test #1044 , which makes resources more limited.

[sophia-guo]

That is saying for target x64Linux -> hotspot-jfr, test_list should not include sanity.external

[sophia-guo]

Close and opened in adoptium/temurin-build#1468