remove superfluous browser-shim lines from package.json

[jspaaks]

• [x] https://github.com/nlesc-sherlock/spiraljs/blob/49b660dd0f0179b20ba85a47e9647c30af77c833/package.json#L91
• [x] https://github.com/nlesc-sherlock/spiraljs/blob/49b660dd0f0179b20ba85a47e9647c30af77c833/package.json#L93
• [x] https://github.com/nlesc-sherlock/spiraljs/blob/49b660dd0f0179b20ba85a47e9647c30af77c833/package.json#L96
• [x] https://github.com/nlesc-sherlock/spiraljs/blob/49b660dd0f0179b20ba85a47e9647c30af77c833/package.json#L98

[jspaaks]

should be solved by PR #62