Change bp2build to reflect arm_isa refactor

We now rely on a `with_features` clause to effectively disable
thumb when `arm_isa_arm` is enabled. Thus, we no longer need to
add `-arm_isa_thumb` in bp2build.

Bug: 261733820
Test: Unit tests
Change-Id: I9da85834855b297a75097ce3a11e101f55bd53f4

bp2build/cc_library_conversion_test.go

@@ -3219,10 +3219,7 @@ func TestCcLibraryWithInstructionSet(t *testing.T) {
 `,
 		ExpectedBazelTargets: makeCcLibraryTargets("foo", AttrNameToString{
 			"features": `select({
-        "//build/bazel/platforms/arch:arm": [
-            "arm_isa_arm",
-            "-arm_isa_thumb",
-        ],
+        "//build/bazel/platforms/arch:arm": ["arm_isa_arm"],
         "//conditions:default": [],
     })`,
 			"local_includes": `["."]`,