detector/bitbucket: Remove support for hg URL schema (#280)

This change removes hg as a supported schema for the BitBucket detector as
Mercurial enabled repositories are not longer supported by BitBucket.

Official announcement at https://bitbucket.org/blog/sunsetting-mercurial-support-in-bitbucket.

Test results before change
```
--- FAIL: TestHgGetter_DetectBitBucketDetector (0.52s)
    get_hg_test.go:157: err: unknown BitBucket SCM type:
--- FAIL: TestBitBucketDetector (0.61s)
    detect_bitbucket_test.go:51: err: unknown BitBucket SCM type:
FAIL
FAIL    github.com/hashicorp/go-getter/v2       6.240s
```

Test results after change
```
ok      github.com/hashicorp/go-getter/v2       6.102s
```

detect_bitbucket.go

@@ -58,8 +58,6 @@ func (d *BitBucketDetector) detectHTTP(src string) (string, bool, error) {
 		}
 
 		return "git::" + u.String(), true, nil
-	case "hg":
-		return "hg::" + u.String(), true, nil
 	default:
 		return "", true, fmt.Errorf("unknown BitBucket SCM type: %s", info.SCM)
 	}