Remove coverage_flags from min Bazel version CI tasks

The --instrumentation_filter=^// flag doesn't work correctly with
Bazel 7.4.1 for Rust coverage collection. Remove it from the
ubuntu1804 tasks so they use the default filter, which works.

Author: Tamas Vajk

.bazelci/presubmit.yml

@@ -292,7 +292,6 @@ tasks:
     build_targets: *default_linux_targets
     test_targets: *default_linux_targets
     coverage_targets: *default_linux_targets
-    coverage_flags: *coverage_flags
     post_shell_commands: *coverage_validation_post_shell_commands
   ubuntu1804_with_aspects:
     name: "Min Bazel Version With Aspects"
@@ -303,7 +302,6 @@ tasks:
     test_targets: *default_linux_targets
     build_flags: *aspects_flags
     coverage_targets: *default_linux_targets
-    coverage_flags: *coverage_flags
     post_shell_commands: *coverage_validation_post_shell_commands
   ubuntu2204_min_rust_version:
     name: "Min Rust Version"