Remove unnecessary check of policy CMP0049.  The policy can never
trigger on our internally-generated name because it has no variable
references.

The rename in commit 0ed5ce4c (cmTarget: Rename AddSource method for
backward compatibility., 2014-03-17, v3.1.0-rc1~688^2~17) made it look
like this code path depended on CMP0049.  Then commit 0e1faa28
(cmMakefile: Separate custom command setup from actual creation,
2019-09-14, v3.16.0-rc1~85^2) and commit ea1bed34 (cmMakefile: Extract
utilities used for creation of custom commands, 2019-09-21,
v3.16.0-rc1~52^2~1) built additional infrastructure to thread that
dependence through the call stack.  Remove it all.