cmd/gomobile: remove unnecessary IsDir check
cmd/gomobile: remove unnecessary IsDir check

The check when looking for android platform is unnecessarily stringent
in that the error would be caught by the call to os.Stat, and
it fails when the android platform folder is a symlink,
which is the case when developing on NixOs.

Change-Id: I07a96c969ffa29dea8bf8e353264e8597dcdc550
GitHub-Last-Rev: c684b80402ad30ad52eb854951aab69657ad7a5d
GitHub-Pull-Request: golang/mobile#55
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/267081
Trust: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
Trust: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Run-TryBot: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>

golang/mobile,golang/mobile,golang/mobile,golang/mobile