Simplify backtrace_functions GUC code

Hi,

While working on [1], I noticed that the backtrace_functions GUC code
does its own string parsing and uses another extra variable
backtrace_function_list to store the processed form of
backtrace_functions GUC.

I think the code can be simplified a bit by using
SplitIdentifierString like in the attached patch. With this,
backtrace_function_list variable and assign_backtrace_functions() will
go away.

Thoughts?

[1] https://www.postgresql.org/message-id/CALj2ACXG1wzrb8%2B5HPNd8Qjr1h8GYkW-ijWhMYr2Y8_DzOB-%3Dg%40mail.gmail.com

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com