Remove more redundant code

* Checking for apex sessions in makeSessionActiveLocked is redundant
cause we already ensure in handleInstall() that no apex session manages
to move forward.
* The package name never gets used by the observer in handleReturnCode.
This is the same observer that we created in makeSessionActiveLocked,
which disregards the sent package name. We only care about the return
code.
* mRet is assigned but never used in MultiPackageInstallParams

Bug: 136257624
Test: atest GtsSecurityHostTestCases
Test: atest StagedInstallTest
Change-Id: I28db925d174959914b513479f0747ce4d7f6fc0b