core: Remove template parameters of in-class constructors

Lars Toenningrequested to merge
ltoenning/ns-3-dev:fix_dr_2237 into master

With C++20, these template parameters of constructor declarations or definitions inside a class declaration are not possible anymore. See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100997#c1

Removing them should be backwards compatible with C++17.

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