Fix Issue #6073- "[PM] Installer steals file associations with no warning (for a few users)"

Removed the installing of registry key `PerceivedType` which appears to be overriding the associated icon set by other apps on Windows 7.

See [@SAPlayer's comment here](https://github.com/adobe/brackets/issues/6073#issuecomment-46685036) for more details.

Note: this only affects Windows 7 apparently.  The reported problem does not exist in Windows 8.x.