Revert r45361 for now "(bug 16084) Default memory limit should be increased"

Revert r45361 for now "(bug 16084) Default memory limit should be increased"
The logic is wrong and will fail to raise the limit to the new limit if the server's default is between 20 and 50.
Further, the whole logic is crappy; we should be doing the check at runtime rather than at install time, since the server's settings may change after we install.