zap unused _MODQT*_PKGMATCH to fix make show=... error

$ make show=PKGNAME
: not found
*** Warning in /p/net/tdesktop: "" returned non-zero status (/usr/ports/x11/qt6/qt6.port.mk:60)
tdesktop-4.9.4

Same error for any other qt6 port, qt6 only differs in file:line.

First time I see those errors, so I suspect a recent change unearthed
them, but I couldn't find anything in the logs.

!= expands and passes the output (empty string) to sh(1), which makes no
sense to me; nothing I can see uses theses variables, so remove them.

Am I missing something?
OK?

Index: x11/qt5/qt5.port.mk
===================================================================
RCS file: /cvs/ports/x11/qt5/qt5.port.mk,v
retrieving revision 1.30
diff -u -p -r1.30 qt5.port.mk
--- x11/qt5/qt5.port.mk 11 Mar 2022 20:16:58 -0000 1.30
+++ x11/qt5/qt5.port.mk 1 Sep 2023 16:34:11 -0000
@@ -182,7 +182,6 @@ SUBST_VARS += MODQT5_COMMENT
MODQT5_VERSION = ${QT5_VERSION}
MODQT_VERSION ?= ${MODQT5_VERSION}

-_MODQT5_PKGMATCH !=
show_deps: patch
@cpkgs=$$(echo ${_MODQT5_CMAKE_PKGS:NQt5} | sed 's/ /|/g'); \
find ${WRKSRC} \( -name '*.pr[iof]' -o -iname '*cmake*' \) -exec \
Index: x11/qt6/qt6.port.mk
===================================================================
RCS file: /cvs/ports/x11/qt6/qt6.port.mk,v
retrieving revision 1.6
diff -u -p -r1.6 qt6.port.mk
--- x11/qt6/qt6.port.mk 6 May 2022 18:59:58 -0000 1.6
+++ x11/qt6/qt6.port.mk 1 Sep 2023 16:33:56 -0000
@@ -57,7 +57,6 @@ ONLY_FOR_ARCHS ?= ${CXX11_ARCHS}
MODQT6_VERSION = ${QT6_VERSION}
MODQT_VERSION ?= ${MODQT6_VERSION}

-_MODQT6_PKGMATCH !=
show_deps: patch
@cpkgs=$$(echo ${_MODQT6_CMAKE_PKGS:NQt6} | sed 's/ /|/g'); \
find ${WRKSRC} \( -name '*.pr[iof]' -o -iname '*cmake*' \) -exec \