[lldb] Remove FormattersContainer's name member

Summary:

FormattersContainer currently has an unused `m_name` member. Usually LLDB allows
giving objects names, but for the FormattersContainer it seems excessive. There
are only 4 FormattersContainer variables in LLDB and they are not usually passed
around, so one can always just go up a few frames when debugging to find out
which FormattersContainer you're dealing with.

Reviewers: mib, davide

Reviewed By: mib

Subscribers: JDevlieghere

Differential Revision: https://reviews.llvm.org/D84154