[tests] remove stale code in test bootstrap script

[Irving-cl]

This PR removes the if branch of BUILD_TARGET of 'otbr-dbus-check' in the bootstrap script.

In our CI, there is no such target now. All the targets:

➜  ot-br-posix git:(clean_up_bootstrap) ✗ grep -rnI --exclude-dir={build,third_party} BUILD_TARGET ./.github/
./.github/workflows/openwrt.yml:52:        BUILD_TARGET: "openwrt-check"
./.github/workflows/build.yml:50:      run: BUILD_TARGET=pretty-check tests/scripts/bootstrap.sh
./.github/workflows/build.yml:62:      BUILD_TARGET: check
./.github/workflows/build.yml:85:      BUILD_TARGET: check
./.github/workflows/build.yml:104:      BUILD_TARGET: script-check
./.github/workflows/build.yml:120:      BUILD_TARGET: scan-build
./.github/workflows/build.yml:135:      BUILD_TARGET: package
./.github/workflows/build.yml:152:      BUILD_TARGET: check
./.github/workflows/meshcop.yml:57:        BUILD_TARGET: "meshcop"
./.github/workflows/docker.yml:56:        BUILD_TARGET: "docker-check"
./.github/workflows/raspbian.yml:49:      BUILD_TARGET: raspbian-gcc
./.github/workflows/documentation.yml:44:      BUILD_TARGET: check
./.github/workflows/ncp_mode.yml:52:        BUILD_TARGET: ncp_mode

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.58%. Comparing base (2b41187) to head (580b98c).
Report is 935 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2691       +/-   ##
===========================================
- Coverage   55.77%   43.58%   -12.20%     
===========================================
  Files          87      108       +21     
  Lines        6890    13359     +6469     
  Branches        0      963      +963     
===========================================
+ Hits         3843     5822     +1979     
- Misses       3047     7231     +4184     
- Partials        0      306      +306     

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[superwhd]

LGTM 👍