Disable inconsistent second-level pseudo-elements.

[xavierholt]

This is a "fix" for #137.

I added code to differentiate mesylate and acetyl pseudo-elements. This worked for the test case given in #137, but further testing showed it to be fairly unreliable; it often wouldn't correctly detect either group. So I've disabled the entire second pass of pseudo-element detection until we can add something more robust. The result is that mesylate now draws as H₃CO₂S or SO₂CH₃, which should be acceptable for now.

The test SMILES from the original bug report:

C1=C[N](N=C1)C2CN([S](C)(=O)=O)C2

Another one I was using:

CCCC(S(=O)(=O)C)CC(C(C)=O)CCCC

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