Remove redundant assignment in CreateWorkExprContext

Hi,

While discussing [1], I was reading execUtils.c, then I noticed this 
redundant local variable assignment in CreateWorkExprContext(). The 
attached patch fixed that.

[1]: 
https://www.postgresql.org/message-id/CAA4eK1+SEus_6vQay9TF_r4ow+E-Q7LYNLfsD78HaOsLSgppxQ@mail.gmail.com

Best regards,

--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/