[Lint] Remove clang-tidy suppression for _-prefix

Part of OpenAssetIO#1091. Previous versions of clang-tidy would incorrectly flag
the `_1`, `_2`, etc placeholder macros from the Trompeloeil library as
violating our naming convention. This no longer seems to be the case for
clang-tidy v19.

Signed-off-by: David Feltell <david.feltell@foundry.com>

Author: feltech

.clang-tidy

@@ -64,11 +64,6 @@ CheckOptions:
     - { key: readability-identifier-naming.TemplateParameterCase,  value: CamelCase  }
     - { key: readability-identifier-naming.FunctionCase,           value: camelBack  }
     - { key: readability-identifier-naming.VariableCase,           value: camelBack  }
-    # Allow _1, _2, etc as variable (placeholder) names - used for
-    # Trompeloeil mock setup.
-    # TODO(DF): This should be placed in `tests/.clang-tidy` but that
-    #   doesn't work. Reported upstream: https://github.com/llvm/llvm-project/issues/54781
-    - { key: readability-identifier-naming.VariableIgnoredRegexp,  value: "^_[0-9]+$"}
     - { key: readability-identifier-naming.ClassMemberCase,        value: camelBack  }
     - { key: readability-identifier-naming.PrivateMemberSuffix,    value: _          }
     - { key: readability-identifier-naming.ProtectedMemberSuffix,  value: _          }