Summary:
For a while, yb_fdw.c has been mostly dead code, the only function being
used being ybcGetForeignRelSize.  Move that function to yb_scan.c, do
some other fdw-related disconnects, and delete the dead files yb_fdw.c
and yb_fdw.h.

Jira: DB-19034

Test Plan:
Jenkins

Close: #29257

Reviewers: amartsinchyk, gkukreja

Reviewed By: gkukreja

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D48072