[srp] Fix SRP buffer sizes (#18139)

The code for calculating buffer sizes used for SRP service
data wrongly assumed that commissionable services are used
only when the extended discovery is enabled. This wasn't
caught before because the extended discovery was enabled by
default on all Thread platforms (until recently).

Signed-off-by: Damian Krolik <[email protected]>

src/platform/OpenThread/GenericThreadStackManagerImpl_OpenThread.h

@@ -171,21 +171,13 @@ class GenericThreadStackManagerImpl_OpenThread
         static constexpr uint8_t kDefaultDomainNameSize  = 20;
         static constexpr uint8_t kMaxDomainNameSize      = 32;
 
-#if CHIP_DEVICE_CONFIG_ENABLE_EXTENDED_DISCOVERY
-        // Thread supports both operational and commissionable discovery, so buffers sizes must be worst case.
+        // SRP is used for both operational and commissionable services, so buffers sizes must be worst case.
         static constexpr size_t kSubTypeMaxNumber   = Dnssd::Common::kSubTypeMaxNumber;
         static constexpr size_t kSubTypeTotalLength = Dnssd::Common::kSubTypeTotalLength;
         static constexpr size_t kTxtMaxNumber =
             std::max(Dnssd::CommissionAdvertisingParameters::kTxtMaxNumber, Dnssd::OperationalAdvertisingParameters::kTxtMaxNumber);
         static constexpr size_t kTxtTotalValueLength = std::max(Dnssd::CommissionAdvertisingParameters::kTxtTotalValueSize,
                                                                 Dnssd::OperationalAdvertisingParameters::kTxtTotalValueSize);
-#else
-        // Thread only supports operational discovery.
-        static constexpr size_t kSubTypeMaxNumber    = Dnssd::Operational::kSubTypeMaxNumber;
-        static constexpr size_t kSubTypeTotalLength  = Dnssd::Operational::kSubTypeTotalLength;
-        static constexpr size_t kTxtMaxNumber        = Dnssd::OperationalAdvertisingParameters::kTxtMaxNumber;
-        static constexpr size_t kTxtTotalValueLength = Dnssd::OperationalAdvertisingParameters::kTxtTotalValueSize;
-#endif
 
         static constexpr size_t kServiceBufferSize = Dnssd::Common::kInstanceNameMaxLength + 1 + // add null-terminator
             Dnssd::kDnssdTypeAndProtocolMaxSize + 1 +                                            // add null-terminator