Remove ECLIPSE_HOME from PDE and instead let it be provided by JDT

[laeubi]

See discussion here:

• Make PDE builds standalone #1136 (comment)

To give some context:

• Seems not used anywhere beside the default location
• org.eclipse.debug.internal.core.SystemVariableResolver even resolves it as a system variable
• Effectively can only be different with a target platform whose first location is a profile location, in all other cases most likley gives wrong results and unexpected behavior
• there is already a variable the do the same named ${target_home}

I therefore would suggest to:

1. Remove ECLIPSE_HOME from PDE and instead provide it by JDT
2. Instead provide TARGET_HOME just in case anyone anywhere is still needing exactly this behavior