gh-38306: Reduce outdated `gdb` and `valgrind` packages to dummy packages
    
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- As discussed in #30158.

Resolves #30158.

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### ⌛ Dependencies

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URL: #38306
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee

Author: Release Manager

build/pkgs/gdb/SPKG.rst

@@ -18,8 +18,3 @@ Upstream Contact
 ----------------
 
 http://www.gnu.org/software/gdb/
-
-Special Update/Build Instructions
----------------------------------
-
-Current version needs makeinfo installed to build successfully.

build/pkgs/gdb/checksums.ini

@@ -1 +1 @@
-experimental
+optional

build/pkgs/gdb/dependencies

@@ -4,9 +4,6 @@ valgrind: Memory error detector, call graph generator, runtime profiler
 Description
 -----------
 
-This is an optional spkg. It supports Linux on x86, x86-64, ppc, ppc64
-and ARM as well as Darwin (Mac OS X 10.5 and 10.6) on x86 and x86-64.
-
 Valgrind is an instrumentation framework for building dynamic analysis
 tools. There are Valgrind tools that can automatically detect many
 memory management and threading bugs, and profile your programs in
@@ -18,10 +15,7 @@ branch-prediction profiler, a call-graph generating cache and
 branch-prediction profiler, and a heap profiler. It also includes three
 experimental tools: a heap/stack/global array overrun detector, a second
 heap profiler that examines how heap blocks are used, and a SimPoint
-basic block vector generator. It runs on the following platforms:
-X86/Linux, AMD64/Linux, ARM/Linux, PPC32/Linux, PPC64/Linux,
-S390X/Linux, ARM/Android (2.3.x), X86/Darwin and AMD64/Darwin (Mac OS X
-10.6 and 10.7).
+basic block vector generator.
 
 License
 -------
@@ -35,20 +29,3 @@ Upstream Contact
 
 -  http://www.valgrind.org/
 -  valgrind-user, valgrind-devel mailing lists
-
-Dependencies
-------------
-
--  None
-
-
-Special Build Instructions
---------------------------
-
--  To build on OS X, you need to use Apple's compiler. FSF GCC is
-   unsupported.
-
-Patches
-~~~~~~~
-
--  None.