[IMP] purchase: remove double context key

Before this commit the purchase_rfq action had two context keys
defined. One being empty {} and one with quotation_only set to
True. After this commit there is just one context (the one that is
actually used and applied) on the action.

Empty key added in 12.0 at 6138686d
Second key added in 13.0 at b5cea7e2

closes odoo/odoo#44990

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>