Revert "aspell: store dictionaries in buffer local variable "aspell_dict""

This reverts commit 8b61a557640e62e67f6eb7349c3a12cca9b43b5e.

The local variable in buffers introduced new display bugs and would need
some extra code and optimizations to fix.
This will be reintroduced after the release of version 0.4.0.
aspell: fix spellers used when switching buffer

The bug happened when two buffers have different number of dictionaries and start
with same dictionaries. For example buffer #1 with [en,fr] and buffer #2 with [en].
When switching from buffer #1 to buffer #2, aspell does not use only [en], but [en,fr].