Revert "Exit with error if no filename given to -e switch"

Revert "Exit with error if no filename given to -e switch"

This reverts commit 6df4a7bb4018c7e13e191e59fdd5d9f768ce5592.

As Vincent points out, this commit is not correct. For one reason,
it would incorrectly exit with error for the following syntax:

lyx myfile.lyx -e pdf

I will look for a correct solution to the problem specified in the
message of the reverted commit.