Don't override ApiException::__toString

[dasl-]

Since ApiException overrides the __toString method, it does not behave as one might expect other Exception subclasses to behave. Stop overriding it so that we have more "conventional" php behavior. In particular, the backtrace will now be part of the string.

Before:

php > require 'src/ApiException.php';
php > $e = new \Google\ApiCore\ApiException('test_message', 1, 'test_status');
php > var_dump("$e");
string(42) "Google\ApiCore\ApiException: test_message
"

After:

php > $e = new \Google\ApiCore\ApiException('test_message', 1, 'test_status');
php > var_dump("$e");
string(84) "Google\ApiCore\ApiException: test_message in php shell code:1
Stack trace:
#0 {main}"

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[bshaffer]

This change will be made as part of our upcoming v2 release