Remove needless type casting.

(Imported from upstream's 55abd566ea02cd21f7bff60db41f97bddad31496.)

Change-Id: Ic48c3d2f6598cbf92ebda0f99e011647eb33f9f4
Reviewed-on: https://boringssl-review.googlesource.com/17104
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/x509/by_file.c

diff --git a/crypto/x509/by_file.c b/crypto/x509/by_file.c
index ebeb72e..2bec572 100644
--- a/crypto/x509/by_file.c
+++ b/crypto/x509/by_file.c
@@ -89,12 +89,12 @@
                         long argl, char **ret)
 {
     int ok = 0;
-    char *file;
+    const char *file;
 
     switch (cmd) {
     case X509_L_FILE_LOAD:
         if (argl == X509_FILETYPE_DEFAULT) {
-            file = (char *)getenv(X509_get_default_cert_file_env());
+            file = getenv(X509_get_default_cert_file_env());
             if (file)
                 ok = (X509_load_cert_crl_file(ctx, file,
                                               X509_FILETYPE_PEM) != 0);