Don't set LD_LIBRARY_PATH by default on Linux (#28354)

`LD_LIBRARY_PATH` can break system executables (e.g., when an enviornment is loaded) and isn't necessary thanks to `RPATH`s.  Packages that require `LD_LIBRARY_PATH` can set this in `setup_run_environment`.

- [x] Prefix inspections no longer set `LD_LIBRARY_PATH` by default
- [x] Document changes and workarounds for people who want `LD_LIBRARY_PATH`

etc/spack/defaults/cray/modules.yaml

@@ -13,9 +13,4 @@
 # Per-user settings (overrides default and site settings):
 #   ~/.spack/modules.yaml
 # -------------------------------------------------------------------------
-modules:
-  prefix_inspections:
-    lib:
-      - LD_LIBRARY_PATH
-    lib64:
-      - LD_LIBRARY_PATH
+modules: {}

etc/spack/defaults/linux/modules.yaml

@@ -13,9 +13,4 @@
 # Per-user settings (overrides default and site settings):
 #   ~/.spack/modules.yaml
 # -------------------------------------------------------------------------
-modules:
-  prefix_inspections:
-    lib:
-      - LD_LIBRARY_PATH
-    lib64:
-      - LD_LIBRARY_PATH
+modules: {}