Fix warning about implicitly declared assignment operator

[coldfix]

Hi,

this fixes a compiler warning when using the assignment operator on a DateTime object. The auto-generated DateTime& operator=(const DateTime&) is deprecated due to presence of a user provided copy constructor.

error: implicitly-declared 'constexpr DateTime& DateTime::operator=(const DateTime&)' is deprecated [-Werror=deprecated-copy]
  133 |     now = rtc.now();
      |                          ^~~~~~~~~~~~
/home/thomas/Arduino/libraries/RTClib/src/RTClib.h:148:3: note: because 'DateTime' has user-provided 'DateTime::DateTime(const DateTime&)'
  148 |   DateTime(const DateTime &copy);
      |   ^~~~~~~~

(here as error, because I am compiling with -Werror)

This PR solves the issue by removing the user-provided copy constructor, therefore using auto-generated copy constructor (which is identical anyway). In this case, there is no problem with also auto-generating the assignment operator.

I can change the PR to instead add a user-defined assignment operator if you prefer that approach.

Btw, thanks for the awesome library and all of the other modules!

Best, Thomas