pgsql: Don't define FRONTEND for initdb

Don't define FRONTEND for initdb

No headers requiring FRONTED to be defined are included as of af1a949109d.

Since this is the last user of (contrib|frontend)_defines in Mkvcbuild.pm,
remove them.

Discussion: https://postgr.es/m/20220820194550.725755r6fj2ro3rx@awork3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/06e3559bade1da89feb1a53d517c991fb4f15272

Modified Files
--------------
src/bin/initdb/Makefile     |  2 +-
src/tools/msvc/Mkvcbuild.pm | 26 ++++++++------------------
2 files changed, 9 insertions(+), 19 deletions(-)