Revert "Guard diag in avifDiagnosticsClearError()"

This reverts commit 173961dac588d5b7cf0d6749d1aada9050b6197a in the
1.0.0 release, to give us more time to re-evaluate
https://github.com/AOMediaCodec/libavif/pull/1496.

src/diag.c

diff --git a/src/diag.c b/src/diag.c
index 8512063..8537018 100644
--- a/src/diag.c
+++ b/src/diag.c
@@ -9,9 +9,7 @@
 
 void avifDiagnosticsClearError(avifDiagnostics * diag)
 {
-    if (diag) {
-        *diag->error = '\0';
-    }
+    *diag->error = '\0';
 }
 
 #ifdef __clang__
@@ -20,6 +18,7 @@
 void avifDiagnosticsPrintf(avifDiagnostics * diag, const char * format, ...)
 {
     if (!diag) {
+        // It is possible this is NULL (e.g. calls to avifPeekCompatibleFileType())
         return;
     }
     if (*diag->error) {