field-class.hpp: remove redundant template parameters

field-class.hpp: remove redundant template parameters

C++ Standard excerpt:
  14.6.1 Locally declared names [temp.local]
  Like normal (non-template) classes, class templates have an
  injected-class-name (Clause 9). The injected-class-name can be used
  with or without a template-argument-list. When it is used without a
  template-argument-list, it is equivalent to the injected-class-name
  followed by the template-parameters of the class template enclosed in
  <>. When it is used with a template-argument-list, it refers to the
  specified class template specialization, which could be the current
  specialization or another specialization.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I1c0c104431f7cf57806a66ed1bce964c007f2c47
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7527
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10762
Tested-by: jenkins <jenkins@lttng.org>
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>