remove year 2038 check

https://gerrit.openbmc-project.xyz/c/openbmc/bmcweb/+/49188 resolves
the year 2038 problem in BMCWeb. There's no need to check it in
cert-manager anymore. However, the current Cert interface can't take
certificate whose NotBefore is before the Unix Epoch given the
timestamp is uint64_t. So this change adds the check to return
errors in this case.

This change also fixed the existing issue of setting unix epoch.

TESTED: unit tests + QEMU
1. added a cert that's valid from 1970/01/01 to 9999/12/31 into
 unit tests
2. tested the dbus properties in QEMU after installing the above
 cert;

```
.ValidNotAfter                        property  t         253402300799                             emits-change writable
.ValidNotBefore                       property  t         0                                        emits-change writable
```
This is expected.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: Idc6b7721fc84b6b9022467e6b0c9e1984f682912
diff --git a/certificate.hpp b/certificate.hpp
index 2a09e69..27ac488 100644
--- a/certificate.hpp
+++ b/certificate.hpp
@@ -120,16 +120,16 @@
 
   private:
     /**
-     * @brief Return error if ceritificate expiry date is gt 2038
+     * @brief Return error if ceritificate NotBefore date is lt 1970
      *
-     * Parse the certificate and return error if certificate expiry date
-     * is gt 2038.
+     * Parse the certificate and return error if certificate NotBefore date
+     * is lt 1970.
      *
      * @param[in] cert  Reference to certificate object uploaded
      *
      * @return void
      */
-    void validateCertificateExpiryDate(const X509_Ptr& cert);
+    void validateCertificateStartDate(const X509_Ptr& cert);
 
     /**
      * @brief Populate certificate properties by parsing given certificate file